Samsung Dock "Speaker Out"

Samsung Dock "Speaker Out"

So when I purchased my GS3, the guy I bought it from gave me a plethora of Samsung accessories, one being the Samsung dock that flips open and has a 3.5mm jack on the back of it. It says "speaker out," but when I plug in to my sound system (or anything for that matter) it either doesn't recognize that I have it plugged in or I am not doing something correctly... Anyone else having this issue?